This invention relates to a power module and a vehicle-mounted inverter using the power module, and more particularly to a power module having the function of switching and a vehicle-mounted inverter that uses this power module and converts the DC power of the battery into AC power to control the motor which drives the vehicle.
In general, the inverter has the function of converting the DC power supplied from a DC power source into AC power to be supplied to such an AC load as an electric rotary machine, or the function of converting the AC power generated by the electric rotary machine into DC power to be fed back to the DC power source.
The inverter, which fulfills such power conversion functions as described above, includes an inverter circuit composed mainly of semiconductor switching elements. The inverter circuit performs electric power conversion from DC power into AC power or from AC power to DC power, by turning on and off the semiconductor switching elements.
Usually, the interruption of current due to the turnoff of the semiconductor switching elements gives rise to spike-shaped voltages as a result of transient phenomenon caused by the parasitic inductance distributed throughout the circuit. In order to suppress such spike voltages, it is required to provide a smoothing capacitor in the circuit and to reduce the parasitic inductance of the DC circuit. JP-A-2002-34268 discloses a technique to suppress spike voltages by reducing circuit-distributed parasitic inductance. According to JP-A-2002-34268, the parasitic inductance is reduced by shortening the length of wiring conductors connecting the smoothing capacitor and the semiconductor switching elements so that the surging voltages can be suppressed.
The inverter mounted on a vehicle receives DC power from the DC power source mounted on the vehicle and converts the DC power into 3-phase AC power to be supplied to, for example, the electric rotary machine to drive the vehicle. There has recently been an increasing demand for the vehicle-mounted, electric rotary machine of the present day to generate greater torque as compared with the vehicle-mounted, electric rotary machines in their early stage of appearance. To meet such a demand, the vehicle-mounted inverters tend to perform the conversion of ever increasing power.
The inverter mounted on a vehicle is usually operated in a high-temperature environment as compared with an ordinary inverter used for power conversion in industrial machinery operating in a factory. Therefore, it is required for the vehicle-mounted inverter to reduce as much heat generated by itself as possible. Of the heat generated by the vehicle-mounted inverter itself, the greatest portion is the heat generated by the semiconductor switching elements included as main parts in the inverter circuit. Thus, it is strongly desirable to reduce as much heat generated by the switching elements as possible.
With the switching elements mentioned above, the generation of heat increases at the time of change from turn-on to turn-off or from turn-off to turn-on. It is therefore desirable to reduce the heat generation at the time of switching. The first solution to reduce such heat generation is to shorten the time of switching in each switching element.
Further, the second solution to the reduction of the overall heat generation is to prolong the time interval at which the switching elements perform switching actions, that is, to reduce the number of switching operations per unit time. However, too long an interval at which the switching operations take place, may incur a poor precision in control. Therefore, there is a restriction on the extent to which the number of switching operations per unit time is largely decreased.
JP-A-2007-143272 discloses a technique according to which the time required for the switching action of each switching element in the inverter circuit is shortened by reducing the parasitic inductance so that the heat generation per single switching action can be decreased.
In addition to the technique disclosed in JP-A-2002-034268, there has recently been an increasing demand for further decreasing heat generation by decreasing parasitic inductance.
JP-A-2007-143272 discloses the fact that the reduction of parasitic inductance results in the reduction of heat generation per single switching action of each switching element and the technique for reducing parasitic inductance. However, since the vehicle-mounted inverter is usually disposed in a tight space, it must generate less heat and be made smaller in size.
In general, as the amount of power converted by an inverter increases, the size of the inverter tends to increase. Despite this, however, the vehicle-mounted inverter must be built in as small a size as possible even though it must handle an increasing amount of power. For example, it is necessary to devise an artifice for increasing the value of the maximum convertible power per unit volume of inverter.
For this purpose, the reduction of parasitic inductance needs to be compatible with the reduction of size. Here, it should be understood that the reduction in size means to increase the value of the maximum convertible power per unit volume of inverter to the greatest extent possible. Further, the use of low-inductance wiring layout helps decrease the heat generation by each switching element so that the area of semiconductor substrate occupied by each switching element can be decreased, whereby the size of the resulting inverter can be reduced. The realization of small size and low cost helps promote the dissemination of the resulting inverters, and hence the energy conservation and environmental protections.